The present invention relates generally to a color display. More specifically, the invention is directed to a digital convergence apparatus for use in color projection-type television display systems.
As a prior art related to improvement in accuracy of convergence correction around the perimeter of a screen, for example, that described in Japanese Patent Laid-open No. Sho 60-185482 is known. This prior art is devised as follows: in a device that stores correction data corresponding to correction points, which are placed inside the displayed portion (herein referred to as the “display range”) of a horizontal scan line and outside the display range (namely, the non-viewable blanking periods of the horizontal scan line adjacent to the viewable portion), in a memory, and that converts the correction data into analog quantitative data, and that generates a correction waveform of convergence through a low-pass filter (LPF), a cross-hatching pattern, in which an interval between vertical lines in the perimeter of the screen (both right and left edges) becomes closer than that in a central portion of the screen, is displayed on the screen; and extrapolation operation of correction data corresponding to the correction points outside the screen is performed using the convergence correction data that has been manually corrected while watching the vertical lines displayed on both of the right and left edges.
In the above-mentioned prior art, a vertical line of the cross-hatching pattern is added to both of the right and left edges of the screen one by one, and the convergence correction at both of the right and left edges of the screen is performed with reference to the lines. Because of it, the convergence correction in both of the right and left edges of the screen becomes easy. However, if a difference between correction data corresponding to the correction point outside the screen and correction data corresponding to its adjacent correction point inside the screen is large, a correction waveform obtained through the LPF may differ from desired one (to be more specific, a level of the correction waveform corresponding to a certain correction point does not correspond with the correction data of the correction point). For example, the following problem arises: influence of the correction data outside the screen causes a correction waveform in proximity to a starting position of the display range to become smaller than correction data in proximity to its position, resulting in decrease in accuracy of the convergence correction around the perimeter of the screen.
The decrease in accuracy of the convergence correction around the perimeter of the screen is remarkable when performing the convergence correction of a high-definition video signal such as that used in high-definition television broadcasts. This will be described with reference to FIGS. 9 and 10 below.
In general, as shown in FIG. 9, a digital convergence correction circuit, which supports a NTSC video signal, is configured in the following manner: the total number of correction points placed in one horizontal scan line is about 16 (n=16); and the number of correction points in the display range (90% of an effective display range) is about 13 (m=13, m is an integer and m<n). On the other hand, concerning a high-definition video signal such as that used in high-definition television broadcasts, a blanking period is shorter than that of a NTSC video signal (more specifically, an effective display range is wide). Therefore, if the numbers of the correction points for a high-definition video signal are configured to be the same as those for the NTSC video signal (that is to say, n=16 and m=13), as shown in FIG. 10, periods 1001 at right and left edges of the screen are significantly shifted from a range within which the convergence correction data can be set by the manual adjustment. Therefore, there is a possibility of decreasing accuracy of the convergence correction in proximity to the perimeter of the screen.
In order to improve the accuracy of the convergence correction in proximity to the perimeter of the screen, the following method can be considered: the number of the correction points in proximity to a boundary between the display range and the blanking period is increased by increasing the values of n and m, for example, by doubling the values (n=32, m=27). However, if the number of n is doubled, labor at the time of the manual adjustment, the required size of a circuit, and memory capacity will increase greatly, which is not desirable.
In view of the foregoing, the present invention has been made, and with the desire to provide a projection-type television receiver in which accuracy of convergence correction at right and left edges of a screen is improved without greatly increasing the number of correction points.